Abstract/Summary

Aerosols offset a poorly quantified fraction of anthropogenic greenhouse gas warming, and the aerosol impact on clouds is the most uncertain mechanism of anthropogenic climate forcing. Here, observations of a relatively weak average response of liquid cloud water to aerosol perturbations are extended to much larger areas than in previous studies, with the polluted cloud areas covering hundreds by hundreds of kilometers. Polluted clouds detected in satellite images at the global anthropogenic air pollution hot spot of Norilsk, Russia, and at other various major aerosol source regions show close compensation between aerosol-induced cloud water increases and decreases. In the sampled Norilsk cloud perturbations the decrease in LWP offsets 3% of the radiative forcing through the Twomey effect on average. Weak cloud water response on average in large polluted areas is in very good agreement with previous results based on observations of small-scale ship-track-likeindustrial cloud perturbations, helping to reduce the uncertainty associated with the anthropogenic aerosol impacts on clouds.